Nitric oxide (NO) is a radical effector molecule with diverse roles, including neurotransmission, vasodilation, and immune killing. NO is made by several isoforms of NO synthase (NOS). The constitutive NOS isoforms are restricted to endothelial cells and neurons, but the inducible NOS isoform, first found in macrophages, occurs in many cell types and perhaps can be expressed in all types. The activity of the inducible NOS is an order of magnitude greater than that of the constitutive isoforms, and NO produced in large quantities can kill or inhibit the growth of a variety of pathogens, including bacteria, fungi, and parasites. NO is particularly effective in blocking the growth of intracellular pathogens, and since the host response to viral infection includes production of cytokines that induce NOS, NO might also play a role in the immune response to viruses. Coxsackievirus infects the hearts of mice and humans. Although the natural history of coxsackievirus myocarditis in humans is not well defined, in mice the severity and duration of coxsackievirus myocarditis is determined by a variety of factors. The host response to coxsackievirus infection is mediated by lymphocytes, macrophages. To explore the hypotheses that viruses induce NOS and that NO inhibits viral replication in myocarditis, the following issues will be examined. Specific aim #1: How does NO inhibit viral replication in vitro? Preliminary data suggest that NO inhibits viral replication in vitro. The anti-viral mechanism of NO will be explored by examining the effect of NO on various stages of the viral replicative cycle, including virus binding to the host cell, penetration, translation, transcription, and viral exit. Specific aim #2: What is the role of NO in murine viral myocarditis? Preliminary data show that viral infection induces NOS in murine hearts, and that inhibiting NOS permits viral replication to increase. The induction of NOS and production of NO will be explored during the acute phase of myocarditis in B 1O.A mice infected with coxsackievirus B3. The effect of administering NOS inhibitors or NO donors on viral replication, inflammation, and mortality will be measured. These experiments will show whether NO generated during viral infection inhibits viral replication in vivo, damages the host, or both. Specific aim #3: Is NOS induced in human viral myocarditis? Using an antibody to human induced NOS, immunohistochemistry will screen for the expression of NOS in endomyocardial biopsy specimens and explanted hearts from patients with and without myocarditis. These studies will help to define the anti-viral mechanism of NO, and the role of NO in defending the host against viral infections.